A shape memory alloy (hereinafter, abbreviated as “SMA”) has a crystal structure called an austenite phase (parent phase) at a high temperature side higher than a transformation temperature and a crystal structure called a martensite phase at a low temperature side. General metal materials do not return to their shapes before deformation once predetermined external forces are applied thereto. However, even if being deformed in a martensite phase by having a predetermined external force applied thereto, an SMA undergoes a phase transformation from the martensite phase to the austenite phase and returns to its original shape before deformation when temperature reaches a transformation temperature or above. By utilizing this property, actuators using SMAs (shape memory alloy actuators) have been developed.
Here, an actuator, which repeats operations in response to a temperature increase and a temperature decrease, is required to be bidirectional so as to correspond to this temperature transformation. However, the SMA returns to its original shape by being heated, but remains its returned memory shape even if being cooled, which means that the SMA is unidirectional. Thus, in one mode of a shape memory alloy actuator, a bias applying member is provided to apply an external force (bias) for deforming an SMA in a direction different from one direction after shape recovery, whereby two-way drive can be realized. Note that a push-pull method using an SMA is also known.
In a shape memory alloy actuator for moving a movable member using a shape memory alloy member (hereinafter, abbreviated as “SMA member”) by such a bias applying method, the position of the movable member is controlled by a position control device taking advantage of properties shown in FIGS. 18 to 20.
FIG. 18 is a graph showing a relationship between a displacement of the movable member and a resistance value of the shape memory alloy member. A horizontal axis of FIG. 18 represents the displacement and a vertical axis thereof represents the resistance value. FIG. 19 is a graph showing a relationship between the displacement of the movable member and a drive current of the shape memory alloy member. A horizontal axis of FIG. 19 represents the displacement and a vertical axis thereof represents the drive current. FIG. 20 is a graph showing a relationship between an instruction value indicating a position of the movable member as a control target and the resistance value of the shape memory alloy member. A horizontal axis of FIG. 20 represents the instruction value and a vertical axis thereof represents the resistance value. FIG. 21 is a graph showing a relationship between the instruction value indicating the position of the movable member as the control target and the drive current of the shape memory alloy member. A horizontal axis of FIG. 21 represents the instruction value and a vertical axis thereof represents the drive current. FIG. 22 is a graph showing a relationship between the instruction value indicating the position of the movable member as the control target and the displacement of the movable member. A horizontal axis of FIG. 22 represents the instruction value and a vertical axis thereof represents the displacement.
The shape memory alloy actuator that moves the movable member using the SMA member that is, for example, expanded by the bias applying member in the martensite phase and returns to its memory shape in the austenite phase at the transformation temperature or higher reached by current heating, and restricts a movable range of the movable member to a predetermined range is described more specifically below. First of all, in a state of this shape memory alloy actuator in the martensite phase where the SMA member is expanded, a resistance value Rs of the SMA member is a maximum resistance value Rmax as shown in FIG. 18. A displacement of the movable member, in this case, is a minimum displacement Pmin (normally, displacement Pmin=0). A drive current Is of the SMA member is a minimum drive current Imin as shown in FIG. 19. This minimum drive current Imin is normally 0 or such a current value as not to displace the SMA member by a bias.
When the SMA member is current-heated in this state by increasing the drive current Is as shown in FIG. 19, the SMA member gradually contracts against the bias to return to its memory shape. Thus, the displacement, P, of the movable member gradually increases. In this case, the resistance value, Rs, of the SMA member gradually decreases as shown in FIG. 18. Eventually, a limit of the movable range of the movable member is reached and the displacement, P, of the movable member reaches a maximum displacement Pmax. In this case, the resistance value Rs of the SMA member appears to be a minimum resistance value Rmin. When a current is further applied for heating, the drive current Is of the SMA member reaches its maximum drive current Is as shown in FIG. 19. This maximum drive current Is is normally, for example, a maximum current value of a power supply. In this case, since the limit of the movable range of the movable member is already reached, the displacement P of the movable member is the maximum displacement Pmax, but the resistance value Is of the SMA member slightly decreases by a backlash of a moving mechanism or the like and eventually does not vary any longer as shown in FIG. 18.
Due to a correlation between the resistance value of the SMA member and an expansion/contraction amount of the SMA member, there is also a correlation between the resistance value of the SMA member and the displacement of the movable member as described. The position control device used in the shape memory alloy actuator can control the position of the movable member by detecting the resistance value of the SMA member without separately providing a position sensor.
On the other hand, the above movement is described below by way of an instruction value input from an external apparatus (e.g. microcomputer) to the position control device instead of the displacement of the movable member.
At a minimum instruction value Xmin, the resistance value Rs of the SMA member is the maximum resistance value Rmax as shown in FIG. 20 and the drive current Is is the minimum drive current Imin as shown in FIG. 21. As the minimum instruction value Xmin increases, the drive current is controlled by a control circuit such that the resistance value Rs of the SMA member gradually decreases from the maximum resistance value Rmax as shown in FIG. 20 and the drive current Is thereof gradually increases from the minimum drive current Imin as shown in FIG. 21. Eventually, when an instruction value X reaches a maximum instruction value Xmax, the resistance value Rs of the SMA member reaches the minimum resistance value Rmin as shown in FIG. 20.
Here, if there is no limit in the instruction value X generated by the external apparatus, the instruction value X beyond the maximum instruction value is input to the position control device. There may also be a case where the maximum instruction value Xmax is exceeded due to a trouble in the position control device or the like.
In such a case, if the maximum instruction value Xmax is exceeded, the position control device tries to execute a control with a resistance value smaller than the minimum resistance value Rmin of the SMA member as a target. As a result, the resistance value Rs of the SMA member becomes smaller than the minimum resistance value Rmin as shown in FIG. 20, but eventually reaches a minimum limit and does not vary any longer. The drive current Is reaches a maximum drive current Imax as shown in FIG. 21.
Such a movement is as shown in FIG. 22 when being expressed in a relationship between the instruction value X and the displacement P of the movable member. Specifically, the displacement P of the movable member is the minimum displacement Pmin at the minimum instruction value Xmin, increases as the instruction value X increases and is the maximum displacement Pmax at the maximum instruction value Xmin. Thereafter, even if the instruction value X exceeds the maximum instruction value Xmax, the displacement P of the movable member does not vary beyond the maximum displacement Pmax.
As described above, there are cases where there is no limit in the instruction value X generated by the external apparatus and the maximum instruction value Xmax is exceeded due to a trouble in the position control device or the like and, in these cases, the SMA member continues to be current-heated through application of the maximum drive current Imax and the properties of the SMA member are degraded. As a result, the performance of the shape memory alloy actuator is reduced. Thus, the position control device of the shape memory alloy actuator requires a protection function of protecting the SMA member from overheating.
For example, in an actuator utilizing a shape memory alloy disclosed in patent literature 1, a limit determiner and a limit controller are provided, whether or not a predetermined limit condition is satisfied is determined so that a wire member made of a shape memory alloy does not exceed a temperature limit below which the wire member normally operates, and the limit controller executes such a control as to stop current application to the wire member so that the shape memory alloy of the wire member is not overheated if it is determined that the limit condition is satisfied as a result of determination. There are cited two determination methods for determining whether or not the limit condition is satisfied. According to a first determination method, it is determined that the limit condition is satisfied if an amount of power supplied to the wire member reaches a predetermined limit value. A supplied power amount to the wire member is calculated from a current application time to the wire member. According to a second determination method, it is determined that the limit condition is satisfied if a difference between or a ratio of a control value and a predetermined reference value exceeds a predetermined value, wherein temperature is cited as the control value.
Here, according to the control methods disclosed in patent literature 1, the shape memory alloy wire member remains its shape when current application is stopped and the position of the movable member driven by the expansion/contraction of the shape memory alloy wire member remains to be the one when current application is stopped if current application to the wire member made of the shape memory alloy is stopped when the limit condition is satisfied. This results in a position control malfunction of the movable member. Thus, if the limit condition is satisfied, it requires a so-called abnormality processing (error handling) such as notification, for example, from the actuator (drive device) to a user to the effect that the limit condition has been satisfied (abnormal state, error state), thereby prompting the user to handle it or recovery of the position control and redoing of initialization by the actuator.
If such an abnormality processing is handled in the drive device or the position control device of the drive device, a system design of the drive device or position control device becomes complicated. Further, an abnormality processing time, for example, for a control stop, a restart, an operation of controlling the position again and the like is necessary, thereby causing time loss. On the other hand, if the position control is simply restarted without handling the abnormality processing in these devices, the devices operate in a similar manner, with the result that there is a high possibility of entering an abnormal state again and executing no position control to a desired position.